One of the biggest victims of the change looks like being the famed kaizen (step-by-step improvement) system that was pioneered by Toyota in the 1970s and has been treated as gospel by manufacturers throughout Japan ever since.
Sadly it seems that immediately after making the paranoid happy by instituting two-step authentication a pretty massive flaw in Cupertino's system was discovered and first reported by The Verge.
The Upgrade Assistant will walk you through the upgrade process step-by-step, from purchase to download and then to installation of the operating system.